1. Field of the Invention
The present invention relates to a two-barrel carburetor for automobile engines or the like and, more particularly, to an improvement of a diaphragm means for driving a secondary throttle valve.
2. Description of the Prior Art
Recently, automobile engines have incorporated two-barrel carburetors having a primary bore and a secondary bore. The two-barrel carburetor has a primary throttle valve, a primary venturi, a primary fuel nozzle, etc., in the primary bore, while it has a secondary throttle valve, a secondary venturi, a secondary fuel nozzle, etc., in the secondary bore. Said primary throttle valve is directly driven by an accelerator pedal-linked system, while said secondary throttle valve is usually driven by a diaphragm means which responds to the vacuum generated by the intake air flow. The diaphragm means for driving the secondary throttle valve is generally adapted to drive the secondary throttle valve toward its open position, in response to the vacuum supplied from the primary and secondary venturi portions to a diaphragm chamber of the diapragm means, against the biasing action of a diaphragm spring disposed in the diaphragm chamber which drives the secondary throttle valve toward its closed position. When the primary throttle valve is opened more, a greater vacuum is generated by the stream of intake air flowing through the primary venturi portion, said vacuum being supplied to the diaphragm chamber of said diaphragm means for opening the secondary throttle valve. However, since the secondary throttle valve is to be maintained in a closed condition until the primary throttle valve reaches a predetermined open position such as, for example, about 50.degree. of rotation from its full closed position, a cam mechanism is provided between the primary and secondary throttle valves for restraining the secondary throttle valve in its full closed position until the primary throttle valve is opened beyond the predetermined angle. Therefore, it is a necessary condition for the secondary throttle valve to be opened by the diaphragm means that the primary throttle valve should be opened beyond the predetermined angle, i.e., the so-called critical angle.
It has been found that a two-barrel carburetor of the above mentioned type having a secondary throttle valve driven by a diaphragm means, provides improved air intake efficiency and operational co-operation between the primary and secondary throttle valves because it does not require any auxiliary valve in the secondary bore such as required in two-barrel carburetors having a mechanical, rigid connection between the primary and secondary throttle valves. However, when the vehicle is accelerated starting from high speed road load running, especially when, for instance, the transmission is in the high speed shift stage, the venturi vacuum supplied to the diaphram chamber of said diaphragm means is already large enough to open the secondary throttle valve before the primary throttle valve reaches said critical angle and therefore the problem occurs that when the primary throttle valve is opened beyond said critical angle so that the restriction imposed by the cam mechanism is removed, the secondary throttle valve is abruptly opened to a substantial extent directly on the heals of the opening of the main fuel supply relative to the supply of intake air. Such a delay in the fuel supply causes an abrupt change of the engine output torque which generates an unpleasant shock.